What do you think are the best or most important things to do in London, England?

My Children and I are traveling to London enroute to Ghana. We will be in London for 3 full days before we head on to Ghana. What do you think are the most important things to do?? Or places to see in such a short time? We are also looking for great places to eat. Last, can someone suggest a pub or location to watch the US BCS National Football Championship?